Management of battery life in a lap top computer is critical to its successful operation. It will be readily understood that large batteries are not desirable for such use because of size and weight restrictions inherit in such a device. Thus, the present direction of the art is to use a relatively small battery (or batteries) of low size and weight, and then to manage the use of the battery to extend its life and operating time.
A significant problem is that computer manufacturers do not have direct control of the management of one of the largest power consuming components of a lap top computer, i.e., the hard disk drive. A commonly used nickel-cadmium rechargeable battery demonstrates a relatively constant gradual decrease in battery voltage as it discharges, followed by an abrupt and rapid drop in voltage near the discharge point. The relatively constant, gradual discharge is a desirable characteristic for computer use, but as the knee of the discharge curve is approached, it is possible that sudden loss of voltage and unexpected termination of operation of the computer could occur. As a result, the user may experience computer failure and shut-down during critical operation as well as subsequent loss or corruption of data. Thus, for proper operation, the steep portion of the battery discharge curve must be avoided. To accomplish this, a battery voltage monitor is employed to warn of impending approach to the steep portion of the battery discharge curve. The computer manufacturer chooses a conservative voltage trip point V.sub.L such that sufficient battery power remains to invoke a mandatory shut down procedure that cannot be aborted by the user, terminating operation while protecting the computer data.
Obviously, a shut down of this type caused by spurious or transient events is an undesirable condition when sufficient charge remains in the battery to continue operation. It has been demonstrated that a hard disk drive can cause just this situation.
A hard disk drive consumes a maximum amount of power in its startup condition because of the energy required to spin up the disk. Normally, when the spin motor of the disk drive is commanded to spin-up, the current supplied to the motor is at a maximum because of the lack of back electromotive force (BEMF). On startup, motor current is only limited by the winding resistance, saturation inductance and voltage drops in the control circuitry and wiring. This approach allows the spin motor to acquire the desired operational speed in the minimum time, which is particularly desirable in contact-start-stop types of disk drives. However, the resulting motor current may be so great that the computer battery voltage momentarily drops below the computer voltage monitor trip point, causing the computer to initiate a power down sequence that terminates operation.
It has become customary to frequently power down the disk drive when it is not needed by putting it in a "sleep" mode to conserve power consumption. When use of the disk drive is required, it is spun up and the peak power surge can, acting through the internal resistance of the battery, momentarily cause the voltage supply to the computer to drop low enough (below V.sub.L) to trip the low voltage monitor and thereby initiate the shut down mode even though the battery is not at the discharge point. Thus, power management techniques such as "sleep" mode can make false trips even more likely.
In reference to FIG. 1, during the disk spin up procedure, the computer internal monitor may be tripped if the battery has a reduced charge (increasing the internal impedance thereof), or is operating at low temperature (again increasing the internal impedance therein, see FIG. 2). Furthermore, the available voltage (capacity) of such a battery is generally lower at low temperatures (and also lower at higher temperatures), as will be seen in FIG. 3, which plots cell voltage as a function of discharge rate (wherein C is the capacity of the cell in ampere hours) for various temperatures, resulting in further reduction of available voltage during motor spin-up.
One possible approach in dealing with this problem is to use a larger battery with lower internal resistance, which, as mentioned, undesirably increases computer size and weight. Another approach is to avoid spin up cycles of the disk drive by always having the disk drive energized, which is counter-productive to prolonging battery life.